Variable inductance



Nov'. 24, 1925` R. w. cAMFlELD ET Al.

VARIABLE INDUCTANCE Filed July 2s. 192s 2 Sheets-Sheet l w/T/vfss: f

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1,562 619 R. W. CAMFIELD ET AL VARIABLE INDUCTANCE FledJuly 25. 1923 2 Sheets-Sheet 2 M Arron/wry.:

Patented Nov.. 24, 1925.

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RUSSELL W. CAMFIELD, OF BERKELEY, AND ROGER M. WISE, 0F OAKLAND, CALI- FORNIA, ASSIGNORS TO E. T. CUNNINGHAM. OF SAN FRANCISCO, CALIFORNIA.

VARIABLE INDUCTANCEI Application filed July 25, 1923. Serial No. 653,6?1.

To all iii/1.0m t may concern.:

Be it known that we, RUSSELL lV. CAM- rncin and Roonn M. lVrsn, citizens ofthe United States, and residents, respectively., of the cities of Berkeley and of Oakland, both in the county ot' Alameda and State ot' (alit'ornia, have invented a new and useful Variable Inductance, of which the following is a specification.

This invention relates to a device for varying the inductance ot' a circuit, and more particularly to such devices especially adapted for use in high frequency circuits, such as radio telegraphy and telephony.

In such high frequency circuits, variable inductances are very often used to tune circuits so as to be resonant to definite Wave lengths, and thereby to render the transmission or receiving" system selective. lt is non' ivell recognized that the wave lengt-h to which a circuit is resonant is a function ot' the, inductancc as Well as of the capacity in the circuit. lu substantially all types of circuits, an increase in either inductance or capacity causes an increase in the resonant wave length, and vice versa.

In order to permit such variable inducl'anccs to be most adaptable, it is desirable that they cover as large a range as possible. One type of such inductances includes a stationary coil or stator. and a serially connected movable coil or rotor, so arranged that by a relative rotation, the coils may be changed from a coaxial position in which the currents in the turns all. assist each other` in both the stator and t-he rotor, to another coaxial position in which the currents in the coils oppose each other. The ideal condition that can exist for minimum .setting is that the opposed coils exactly neutralize each other magnetically, in which condition there would be zero inductance. On the other hand, the ideal condition for maximum setting is that both coils ybe coupled so closely that all of the flux produced by them is common to both coils. Although for obvious mechanical reasons these conditions can never be fully realized because of the fact that there must be clear- :im-es between the coils, vet when utilized with comparatively loiv Jfrequency currents, they may at least be approximated to a fair degree.

However, when using` such 'a variometer with high frequency circuits, sometimes of the order of a million cycles, another factor enters into the tuning range of the device, which is of considerable importance. This factor is the capacity eftect between the rotor and stator Winding, which effect is in many Ways similar'to that of a capacity connected in parallel to at least some of the turns of the coils. While it is a general object of our invention to extend the range ot the type of variable inductance under consideration, `yet it is a specific object so to correlate the capacity effect and the inductance etlect of the coil as to secure an increasedl range of the instrument for tuning a circuit to be resonant to a desired frequency.

Variometers of the type referred to have been designed and placed on the market, but none. so :tar as We have been able to ascertain, have been purposely arranged to secure the maximum range elects, in the manner that We shall hereinafter describe. These variometers are of especial utility for reception of short Wave length energy, such as broadcasted speech or music, which is now transmitted with the' aid of electromagnetic radiations, between about 200 and 550 meters wave length.y This range is considerably greater than the range heretofore assigned to broadcasting stations, and We have found by actual experiment that substantially all of the variometers now on the market are incapable of covering this entire range, although they were Well suited for the reception of speech or music at or near 360 meters, representingr the former narrow 'Wave band allotted to the broadcasting stations. It is also desirable to extend the range downward to include the band allot-` ted to amateur signaling, which band eX- tends from about 180 lto 220 meters. We have also found that a mere increase in the number of turns: does not materially increase the range, due to the inherent departure from perfect close couplings in the minimum and maximum positions.. The

fremedy as a matter of fact remained hidden for an appreciable period to us, but finally by the recognition of certain. principles hitherto neglected or unrecognized, We are able to extend the range of the variometer Without altering the windings. It is thus another object of our invention to provide a variometer that is especially adapted for the broadcasting Wave lengths and for the amateur Wave lengths.

In the operation of the variometer to vary the Wave length oi' the circuit in which it is included, the rotor, as has been mentioned heretofore, may be turned through 180, from full cumulative relation with the stator, to full diierential relation, corresponding respectively to maximum Wave length setting and to minimum wave length settinfr. It is advantageous to ensure that equal angular movements of the rotor will cause equal variations in the Wave length. In prior devices. this desirable condition is not fulfilled; instead, near the eXtreme positions of the rotor, much larger angular movements have been necessary to secure the same Wave length variation as is secured when the rotor is in or near its medium setting. 4It is another object of our invention to ensure substantially proportional changes in setting of the rotor and in wave length, even near the extreme positions ot the rotor.

Our invention possesses other advantageous features, some ol which with the toregoing, Will be set forth at length in the oln lowing description, where We shall outline in full those forms of the invention which We have selected lor illustration in the drawings accompanying and forming part ot the present specification. Although We have shown in the drawings but a tevv modifications of our invention, We do not desire to be limited thereto, since the invention as expressed in the claims may be embodied in other forms also.

Referring to the drawings:

Figure l is a Wiring diagram oit a. typical form of receiving circuit in which our invention may be incorporated;

Fig. 2 is a cross sectional view, mainly diagrammatic, of a variometer which may incorporate our invention;

Figs. 3 and @C are schematic Wiring dia grams of a variometer Winding, inthe manif mum and minimum Wave length setting respectively, said variometer representing in a. general ivay how, Without making use of the teaching of this invention, the range is made rather small as in prior types of variometers;

Figs. 5 andV (S are schematic wiring dia grams, similar'to Figs. 8 and 4, of one term of variometer Winding incorporating our invention: and

Figs. 7 to 16 inclusive are further schematic diagrams of variometer-'windings incorporating our invention.

In Fig. 1 We illustrate a type of circuit in which the variometer forming the subject matter of our invention may be used. In the present instance a thermionic tube detector circuit for receiving radio oscillations is indicated, but of course the variometer 2l incorporated therein is not in any Way restricted to this particular set of connections. An absorbing circuit, such as antenna 22, coupling coil 23, variable condenser 24, and ground 25. is used to receive the electromagnetic radiations in a well known manner, and to pass them on to the detector circuit. This receiving circuit is tuned to resonance, as by the condenser 24, with the radiations to be received. rlllie detector circuit is coupled to the absorbing circuit, as by the aid ot the coil 2G. The therinionic detector tube 2T, as is usual, has an electron emitting cathode, such as a hot filament 28, heated from the battery 29er other source. The detector 2"( also has a grid or control electrode 30, as Well as an anode 3l. The manner of operation of this type ot detector is now Wellknown, and it is neither necessary nor desirable to refer to it in more than a very general Way. The circuit between the grid 3() and filament 28 is usually termed the in put circuit, and in this case includes the varionieter 2l, the coil 26, and a grid condenser 32 shunted by a high resistance 33. The variometer 2l is used in this circuit forv tuning it to the received radio frequency energy. This variometer is indicated as shunted by a capacity 34, which `ve intend to repre-sent (although impertectly) the capacity eiiect between the windings ci the variometer 2l; this capacity effect, as will be shown later,- piays a very important part in determining 4the range ot the instrument 2l..

'.lhe output circuit for the detector 2( includes some torni translating device, such as the phones 34V; the battery 35 for iinpressing a positive voltage on plate 3i; and a potentiometer 36 'which connects to vthe lament '28 across the battery 29. In general it may be stated. 'that the tube 2i', through its control electrode 30 and grid condenser 32, causes the audiotrequency variations present in the received radio waves, to become impressed and amplified in the output circuit. @t course this circuit may be renlaccd. by any other in `which there is need or varying theA resonant conditions ot one portion thereof, as ot the input circuit'in the present instance.

`One form of the variometer 21 is illustrated on a larger scale in Fig. 2, which although diagrammatic, shows the relation ot the windings in an intermediate setting. In this instance the stator 87 has two Winding sections 38 and 39 which, torni substantially hollowf spherical surfaces. The rotor 54D., which is pivoted on an axis passing through the center ol" these spherical surfaces, also has a pair of Winding sections 41. and 42. 'these coil sections are substantially spherical, and extend inside et the stator coils 38 and 39. There is as small clearance be tween the rotor and stator coils as is mechanically practicable. The rotor 40 may be swung through 180o so that the axis of coils 41 and 42 may be moved from coincidence with that of coils 38 and 39, through a mid-position in which these axes are at right anglesl to each other. to coincidence again.l The shaft 43 may be used for supporting and for effecting its movement. An

intermediate position is shown in the ligure. Appropriate standards, such as the bars 44, are usually provided for supporting the stator 37.

The coil sections 38, 39, 41 and 42 are connected in series, as indicated by the variometer 21 of Fig. 1. The inductanee provided by the windings is a function ot the relative magnetic eects o the currents Howing in the coil sections. But in addition to this effect, there is a capacity ettect of considerable importance that exists between the coils. This eiti'ectl will now be morea thoroughly discussed. Some of the current: that flows to and from the vario'in'eter terminals is used as charging currents for condensers formed by the two adjacent windings. These charging currents act as currents passing in shunt to portions of the windings, and in Fig. 1 as stated heretofore, the effect is diagrammatic-ally illustrated as substantiallv equivalent to the current charging and discharging a condenser 34 in parallel to the variometer coils. Since the relative positions of the variometer coils change when the circuit is being tuned, these current effects also vary. Now it may readily be demonstrated that if a. condenser action is provided in shunt to an in.- ductance, the resultant effect `upon the circuit in which the inductance is present, is to increase the resonant wave length. Furthermore variations in this capacity eti'cct cause corresponding variations in the resonant wave length: in other words, a decreasel in the capacity effect causes decrease in the resonant wave length, and vice versa.V4 In our invention, we attempt to make use of these capacity changes as the variometer coil position is varied, to increase the range of the instrument.

The charging currents have another etiect that must be considered. In order to ap- ]n'oximate the ideal conditions for maximum and minimum inductance etfect, currents in the coil turns should be balanced; that is, opposite turns on the stator and rotor should carry equal currents. The charging currents may disturb this balance in such a Way that, in the minimum position, a considerable .residual stray magnetic` field of both the stator and rotor coils prevents neutralization; while in the maximum inductance position, there is incomplete interlinkagc of the stator and rotor fluxes.

In Figs. 3 and 4 we represent schematically the maximum and minimum positions respectively of a variometer in which no particular attention is paid to the charging current effect. The stator coil is shown as made up ot the two sections 38 and 39, While the are other capacity effects, such as bet'eeen the adjacent turns on the same coil, but we find that these have no appreciable eti'ect even at radio frequency'. In the instant under consideration, the currents are assumed to be iowing downwardly where crosses appear, and upwardly from the plane of the paper where dots appear.

In the discussion of the diagrams that will follow, we shall assume that where the current' that ftlows serially through all of the coil sections 38. 39, 41 and 42 (the inductive current), and the charging currents that are combined with the inductive currents in certain turns of the windings. have an additive ett'ect. In other words, where in a turn we find maximum inductive current and maximum charging current, we assume that the resultant sum of the currents is also a maximum: however, this convention is adhered to simply to render the explanation clear and not too involved. and when this assumption is made, the results deduced are the same as if the phase relations of these currents Were also properly taken into the consideration.

In the maximum inductance position ott` Fig. 3, the left hand terminal ot section 3S forms one of the terminals ot the device. while the right hand terminal of section 3!) forms the other. The section 3S first placed in series with sec-tion 41, as by connection 46: it is to be noted that the rig-nt `hand terminal of section 38 is connected to the left hand terminal of section 4l. Current from section 4l is then led to the left hand terminal of section 42, b v the aid of connection 47. From this section the current is led to the leftliand ternlinal ot' section 39, by the aid ot' the connection 48, and as stated heretofore, the right hand terminal of this section forms one ot the terminals of the variometer. This particular scheme of connection has been illustrated to show a common type in general use. which although having a. restricted wave length range. yet has the morel obvious advantages ot' permitting very convenient connections. The restrlction in wave length is material. hou-- ever, and by the aid ot' our invention we iind by actual trial that we can approximately double the range of the variometer by resorting to other schemes of connections,

uch as are exemplified in the succeeding res.

eturning to a consideration of Fig. 3, the maximum resultant current fiows in the extreme left hand turn of section 38, for here the charging current is a maximum. As the current traverses the succeeding turns, it becomes somewhat diminished, de to the ow of chargingcu'rrent in more and more ofthe condensers45. These charging currents reunite until the resultant current through the right hand turn of section 41 is again a maximum. As regards the relation .of bothsections 38 and 41, it is thus seen that the turn of section 38 which carries the maximum current is' opposite the turn of section 41 that carries the minimum current. The result is a material departure from substantial magnetic balance, and therefore 5 from the maximum inductance which it would be otherwise possible to obtain. To a slight extent this eiect is compensated for by the capacitances 45. But this compensation is small, due to the fact that these capacities are subjected to the voltage across a number of turns that is equivalent merely to one section, or in other words, to substantially one fourth of the tota-l drop across the variometer. Exactly the same situation exists as regards the 'sections 39 and 42. it is thus seen that this scheme of connections leaves much room for improvement so far as the capability of the variometer to respond to long wave lengths is concerned.

Nor is the corresponding minimum position of Fig. 4 appreciably better. 1n this position the coils 41 and 42 have been rotated through 180 so that the currents therein flow in a direction opposite to those in the sections 38 and 39, and furthermore the relative positions ot these rotor coils are interchanged due to this rotation. For minimum results, the wiiidings should be balanced, and the capacity effects between layers should be as low as possible. The maximum resultant current of course exists in the extreme let'hand turn of section 38; the opposite turn of section 42 carries nearly the maximum. current also, lfor it is only one section away from the right hand terminal of the variometer, to which it is connected through connection 48, and the turns of section 39. The amount by which the current .inthe left hand turn of section 42 is less than the maximum is represented by the charging currents of condensers 45 between sections 39 and 41, which charging currents do not iiow through section 42 at all, but instead nd a counterpart in the turns of section 39. As we anaiyze the current relationship between other airs of opposed turns on sections 38 andP 42, substantially the same imbalance exists as between the extreme left hand turns. Furthermore, substantially the same unbalanced relation exists between the opposed turns of sections 39 and 41. It is thus seen that again ideal conditions cannot be attained to secure minimum wave length. The minimum wa`ve length is further raised by the considerable capacitye'ect that exists between the layers. In this case the condensers 45 are subjected to a very large drop-for Aexample between the extreme left hand turns of sections 38 and 42, there is a drop substantially equivalent to `three-fourths of the total drop across the variometer. The same is true as regards the extreme right hand turns of sections 39 and 41. Between other opposed turns the drop is less,v but the average eect is rather high, so that the net result is that a considerable capacity effect exists, and a large deviation from the perfect value of minimum wave length results.

VVe-hav'e gone to some length in explaining the disadvantages that may result from a disregard of the capacity lcurrent effects,

in order to emphasize the remarkably eiiicient' remedies therefor that may be provided by practicing our invention. While it is possible that accidentally connections v ywere made that resulted in the advantages of our invention, et there has been no formulation thereofy such as would render it possible to duplicate the results at will. Recognizing that the capacity current efect results in two disturbing factors-variations iny current flow in diierent parts of the conductor, and increase in wave length dependent upon the drops in potential across these condensers-our aim is to arrange the sections of the windings in such a way that the wave length range of the variometer may not be reduced by the first mentioned eiect, and may loe-increased by the second mentioned eiiect. There are of course a considerable number of modifications that could be described havingthe same end in view. For the present we shall consider the form of connections represented in Fig. 5 as the maximum wave length setting and in Fig. 6 as the minimum wave length setting.

In Fig.L 5 the stator sections 38 and 39 are opposite for maximum setting, to the rotor sections 49 and' 50. Here again the extreme left hand terminal of section 38 forms one of the terminals for the entire variometer, and the extreme ri ht hand terminal of section 39 forms the ot er terminal for the variometer. The connections 51,`

52 and 53 as illustrated place vthe sections 38, 50, 49, and 39 in series relation in the order named. From a consideration of the charging currents for condenser 4 5, it becomes evident that there is some unbalance between the currents flowing in opposite turns, there being minimum resultant current in the extreme iet hand turn of section 4.9, While the cppcsed turn of: section 38 carries the maximum resultant current. However, as we progress toward the right of both sections 38 and 49, this unbalance becomes less and less, until at the right hand turns of these sections, there is substantial equality, since the charging currents between these turns tend to reduce the resultant cur-rent in section 38, and to increase it in 'section 49. Substantially the same effects exist between the sections'f) and 5U. It is thus seen that there is some imbalance, although not very great. However, this imbalance is compensated for quite well by the effect of the condensers 45, across which there exists fully half of the total drop of the variometer. The net result is that the maximum wave length is rather high.

The minimum position of this variometer, as shown in' Fig. 6, is exceptionally favorable. The currents are balanced very perfectly; and fairly low potentials exist across the condensers 45. This results in a very material reduction in the minimum wave length.

We have also found by actual trial that the variation in Wave length as the rotor is moved is substantially proportional to the variation in angular movement when the variometer is connected as illustrated in Figs. 5 and 6. This is of material advantage for accurate settings along the entire range.

In Figs. 7 and 8 another-possible form of connections is illustrated that has marked advantages. In this case the rotor sections 54 and 55 are wound reversely as regards the stator sections 38 and 39, but the current directions are maintained the same in all sections by proper interconnections. In general it may be stated that a reversely 'Wound rotor assists in increasing the wave length range, simply because there is either a better balancing of the currents or else proper use is made of the capacity effect. The embodiment of Figs. 7 and 8 is especially important, since it is a form readily obtainablefrom the usual type, by proper arrangement of the connections. In this form the range is about double that of the old form of Figs. 3 and 4. Connections 56, 57, and 58 are provided for properly connecting the reversed turns so -a's to producey the proper cumulative and differential effects. In Fig. 7 for the maximum position there is n0 great unbalance between the sections as may readily be verified, and in addition, there is a comparatively high potential impressed across the opposed turns, so that the capacity currents are increased, and a high maximum value of wave length is Ithus secured. For example the potential difference at the extreme left hand condenser 45 corresponds to the drop across three fourths of the turns of the device. In the minimum position of Fig. 8, the currents are only slightly unbalanced, but here the potentials across the opposed turns are very low, corresponding to the drop across but one fourth of the total turns, and therefore it is possible to secure a good value for the minimum Wave length.

In the succeeding figures we have omitted the condensers corresponding to condensers 45 in the preceding figures, in order to clarify them. IVe shall give one more example of a variometer in which the rotor sections are connected between the stator sections. In Fig. 9, for maximum position, one of the rotor sections 60 is reversely wound; the other section 59 is wound in the same direction as the stator sections. In this position there is a fair degree of current balance, and comparatively high potentials across the distributed capacities. In the minimum position of Fig. 10, there is unbalancing of currents only in the left hand half of the variometer, and furthermore there are low potentials across the distributed capacities.

In Figs. 11 and 12 We show one form of our invention in which the variometer and the stator sections are connected together directly, as Well as the rotor sections. In the maximum position of Fig. 11 the extreme left hand terminal of section 38 forms one terminal of the variometer, While the right hand terminal of section 61 forms the other terminal. In this rposition the potentials are high and there is no material unbalancing for the currents. In the minimum position of Fig. 12, the currents are well balanced, although the capacity potentials are a little high.

In Figs. 13 and 14 still another form of our invention is illustrated. In the maximum position of Fig. 13, the capacity potentials are high, and there'is little unbalancing. In the minimum position of Fig. 14, although the potentials are somewhat high, the unbalancing is rather small.

In Figs. 15 and 16 we show the last form ofour invention, in which both rotor sections 66 and 67 are reversely Wound when in the maximum position. The terminals of the variometer are thus located on the same side, and corresponding to the left hand terminals ofsections 38 and 66. In the maximum position the currents are very well balanced, and furthermore the potentials for the distributed capacities average a high value. Conditions therefore are very favorable for a long wave length. In the minimum position of Fig. 16 the conditions are fairly good 4for a loW Wave length:-the potentials being low, although there is some unbalance.

By reviewing the connections, it is seen that most favorable conditions for both the maximum and minimum settings cannot be obtained for any one set of connections. The optimum conditionsare that the currents be always balanced, and that the capacity potentials be low for the minimum setting, and high for the maximum setting. To secure these results, the minimum setting should be like that of Fig. 6, and the maximum, like that of Fig. 15. Of course, by varying the connections between minimum and maximum settings, thebenefits of these forms could be obtained,` but Yin general it is not advantageous to provide variable connections. A compromise between these forms may be effected by employing the connections of Figs. 7 and 8, or of Figs. 15 and 16. In these figuresthe only bad feature is the lack of balance in currents for minimum settings. However, it may be readily demonstrated that at tings such an unbalance is not as objectionable as at maximum settings. The net result is that the two forms mentioned offer very material advantages over the-standard and well-known forms. In both these forms the rotors are reversely wound as regards the stator, in the cumulative position; in other words, the currents progress through the coil layers in opposite axial directions, although in the same direction of rotation to produce the cumulative effect. In both there are low potentials at minimum settings across the distributed capacities, and at maximum settings there is substantial balance of the currents.

We claim:

1. A variable inductance coil for use in high frequency circuits, said inductance coil comprising a pair lof relatively movable layers of coil sections, arranged in their extreme positions respectively to have a cumulative and a differential effect, characterized by the fact that connections are provided for placing all of the sections in series, said connections being arranged to provide a comparatively high average potential difference across the layers when in extreme cumulative relation, and a substantially lower average potential difference across the layers when in extreme differential rela- 'tion.

2. A variable inductance coil for use in high frequency circuits, said coil compris-r ing a pair of relatively movable layers of coil sections arranged in their extreme positions respectively to have a cumulative effeet and a differential effect, characterized by the fact that connections are provided for placing all of the sections in series, said connections being so arranged that in the extreme cumulative position the currents flowing in opposed coil turns for charging the capacitance formed between the layers, are substantially balanced, and a relatively high average potential exists between the layers, while in the extreme differential position a relatively low average potential exists between the layers.

minimum set-` 3. In a variable inductance coil for use in high frequency circuits, said coil comprising a pair'or` relatively movable layers of coil sections arranged in their extreme positions respectivelyto have a cumulative effect and a differential effect, means for so connecting the coil sections in series that, in the cumulative position, one of the layers is reversely wound as re ards the other layer, the cumulative effect eing produced by making the connection between the layers extend from the end of one layer to that extremity of the other laye'r which is farthest in an axial direction from the beginning of the first layer.

4. In a variable inductance coil for use in high frequency circuits, said coil comprising a pair of relatively movable layers of coil sections that have appreciable `capacitance eiect between them, said layers being arranged in theirextreme positions respectively to have a cumulative effect and a differential effect, means for connecting the sections in series, comprising means for insuring that a relatively high average otential is .impressed across opposite turns inthe layers in the cumulative' position, and a substantially lower value impressed in the differential position.

5. In a variable inductance coil for use in liigh frequency circuits, said coil `comprising a pair of relatively movable layers of coil sectlons that have appreciable capacitance effect between them, causing the flow of charging currents in paths .parallelin at least some of the turns of the coil, sai

layers being arranged in their extreme positions respectively to have a cumulative effect and a differential effect, means for connecting the sections in series, comprising means for insuring that the resultant currents in opposite turns of the layers, due to the charging currents and the inductive current through the coil, are substantially balanced.

6. In a variable inductance coil for use in high frequency circuits, said coil comprising a pair of relatively movable layers of coil sections that have appreciable capacitance effect between them, causing the ow of charging currents in paths parallelinv at least some of the turns of the coil, sai layers being arranged in their 'extreme positions respectively to have a cumulative effect and a differential eli'ect, means for connecting the sections in series, comprising means for insuring that the resultant currents in o posite turns of the layers, due to the c arging currents and the inductive current through the coil, are substantially balanced, and for insuring that a relatively high average potential is impressed across opposite turns in the layers in the cumulative position, and a relatively low value impressed in the differential position.

7. In a variable inductance coil for use in high frequency circuits, said coil com prising a pair of relatively movable layers of coil sections that have appreciable capacitance effect between them, said layers being arranged in their extreme positions respectively to have a cumulative effect and a differential ei'ect, means for connectingsaid sections in series, comprising a connection from one layer to the other, said connection causing the current in the layers in the,

cumulative position to traverse turns in opposite axial directions, although in the same direction of rotation to produce the cumulative eli'ect.

8. In a variable inductance coil for use in high frequency circuits,`said coil comprising a pair of relatively rotatable coil sections, one Within the other, and arranged by relative rotation through substantially 180 to move the sections from axial cumulative alinement, to axial differential alinement, said sections being so close together that appreciable capacity eiiects exist between them, causing the flow of charging currents in parallel to at least some of the coil turns, means for connecting the sections so that in the cumulative position the directions of Winding for the sections are opposite, the

Iconnectlons however being such that the directions of current flow are the same, to produce the cumulative effect.

In testimony whereof, We have hereunto set our hands.

RUSSELL W. CAMFIELD. ROGER M. WISE. 

